Mixture leaning

6 Answers

1. 
   

   
   

   John D. Collins on Jan 25, 2013

   I will assume by a simple aspirated areo engine you have a carburetor equipped engine with a fixed pitch propeller. My comments are based on that assumption.

   When taxiing, you are using low power and the plugs have a tendency to lead foul with an overly rich mixture. This is true at sea level as well. At taxi RPM’s, the unleaned engine will tend to run a rich mixture. I would recommend that you aggressively lean the engine while taxiing. By this, I mean that you pull the mixture knob out around an inch from the full rich position. Usually this will provide smooth operation. If the engine runs rough because it is too lean, enrichen the mixture to the point where the roughness disappears. At higher altitudes, an unleaned engine may be way too rich and not run, so you should at least lean until smooth operation is achieved and then lean another half inch. By running the engine aggressively leaned, you will reduce the plug fowling problem and if for some reason you don’t set your mixture properly for takeoff, the engine will notify you of this condition when you advance the throttle as it will stumble. If you were to lean the engine less aggressively, the engine might not stumble, but it would run hotter than it should.

   At sea level up to a density altitude of approximately 3000 feet, I would push the mixture full rich for takeoff. At higher density altitudes, I would do a full power run up and set the mixture to achieve the highest RPM as this will give you the most power available.

   As you climb above 3000 MSL, I would lean the engine for the highest RPM.

   At cruise, there are two common mixture settings, best economy and maximum power. To establish maximum power, lean the engine for the highest RPM. For best economy, lean the engine until the onset of roughness, then richen sufficiently to just eliminate the roughness.

   As you descend, you are normally using less power and the mixture setting is not as much of an issue. Most POH will suggest that you enrichen the mixture prior to landing. For density altitudes above 3000 MSL, you can level off and briefly go to full power and set your mixture for the highest RPM. This mixture setting can be used for the landing as it will provide good power in the event of a go around and still provide a lean enough setting so that the engine will run smoothly after landing and taxi back to the ramp. If you intend to depart after a brief stop, you might note how far the mixture is out so that when you start up again, you can pre-set the mixture to approximately the same initial setting.

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   khoda bakhsh on Jan 25, 2013

   Thanx for the detailed information. If the aircraft has constant speed unit , what is its relation,?

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3. Best Answer
   
   

   
   

   John D. Collins on Jan 25, 2013

   If you have a constant speed prop, then you can’t use the peak RPM while leaning. Usually such aircraft will have an EGT available. With the EGT, leaning is accomplished by leaning until the EGT temperature peaks, then richening it so that the EGT falls back a certain amount. The POH should indicate the maximum power setting that is permitted when operating at peak EGT, usually this is 65% power for a TCM engine and 75% for a Lycoming engine, but get the data for your specific aircraft out of your POH. Best power at cruise is achieved at 100 to 125 degrees F ( 55 to 75 degrees C) rich of peak. Economy is normally established at either peak EGT or 50 degrees rich of peak. To lean, pull the mixture and watch the EGT to see where it peaks, then note the marking where it peaks and richen it until it falls to the desired difference in temperature. Most single probe EGT systems have a meter that is calibrated in 25 degree increments and have a dial marker that you can set to the peak EGT value for reference. For the takeoff and climb, if you do a full power run up, set the mixture so that it is 150 to 200 degrees rich of peak. Many such engines also have a fuel flow meter with altitude markings that are calibrated for cruise and for climb. You can use the climb values for takeoff and periodically reduce them as you climb.

   If you have a fuel injected engine, then lean of peak may be an option, but it is a very long discussion that I won’t go into here.

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   Chris Carlson on Jan 26, 2013

   http://www.advancedpilot.com/downloads/prep.pdf

   I was doing some research into manifold pressure, and cme across this. An amazing explanation of the three controls for constant speed props- rpm, manifold pressure, and mixture. The bottom section is on mixture. I really can’t stress enough how good this article is.

   However, if you want to save yourself so time, johns explanations work great.

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   khoda bakhsh on Jan 27, 2013

   @ Chris Carlson …WONDERFUL STUFF. HE HAS EXPLAINED EVERY THING RELATED TO PROP IN DETAILS…THUMBS UP

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6. 
   

   
   

   David Brown on Jan 27, 2013

   Rather than using the crude method of peak RPM, if you have an EGT gauge, or better still an engine monitor, fly to a seal level airport and find what the EGT is for THAT airplane at full rich WOT takeoff, as you go through say 5-700 feet.

   Assuming the fuel system is set up right this is the “Target EGT” and you can use that at a high level airport after that, line up, WOT and lean to get the Target EGT as you roll.

   This gives you the same kind of combustion event you would have if you had taken off at sea level and then used the target EGT in the climb.

   http://www.advancedpilot.com would be an excellent source of education. I am involved but do not have a financial gain in the USA, so while you could say I am biased, it is only for technical reasons not financial gain.

   John’s articles in the avweb Pelicans Perch series are good too, but APS course really educate you.

   Sign up for the March seminar, I will be over there again so maybe I will see you there!

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